Conventional non-contact mechanical seals for rotating equipment have generally comprised a rotary sealing ring fixed to a rotating shaft which penetrates a seal case. A holding ring is held movably against the seal case via an O-ring so that the ring may move axially but is free from radial displacement. A stationary sealing ring is fitted on the holding ring by shrink fitting it in an inner hole. Springs are mounted between the seal case and holding ring to bias the stationary sealing ring against the rotary sealing ring via the holding ring. A dynamic pressure generating groove is provided on the rotating side sealed end face to maintain a fluid film in the region between the sealing surfaces thereby sealing a high-pressure fluid zone from a low-pressure (atmospheric) fluid zone.
A conventional seal as described above exhibits certain deficiencies. The rotary sealing ring, the stationary sealing ring and the holding ring are constructed with dissimilar materials with varying thermal expansion coefficients and Young's moduli. For example, the rotary sealing ring is made from extra hard material such as WC, SiC, etc., the stationary sealing ring from comparatively soft materials such as carbon, etc., and the holding ring from metallic materials such as SUS304, Ti, etc. Thermal and pressure strains are produced in the rotary sealing ring, stationary sealing ring, and holding ring by heat generated by operation or system pressure of the equipment. These strains and conditions vary depending on the differences between component materials. In particular, the strain in the holding ring is extremely large due to the component materials. Consequently, in the conventional seal in which the stationary sealing ring is fitted and integrated into the holding ring, the strains interfere with each other at the portion in which the stationary sealing ring comes in contact with the holding ring. Therefore, the stationary sealing ring is strongly subjected to the strain of the holding ring and exhibits a strain condition completely different from its own strain.
Moreover, effectively holding the stationary sealing ring radially in relation to the seal case primarily by the O-ring, which is an elastic member located between the seal case and the holding ring, can result in serious instability and it is possible to displace the stationary sealing ring in the radial direction by pressure fluctuations etc. These factors impair smoothness of the stationary side sealing end face as well as its concentricity and parallelism in relation to the rotating side sealing end face. This results in non-uniformity of the dynamic pressure generated between the sealed end faces and, in an extreme case, gives rise to contingencies such as defective generation of dynamic pressure or localized contact of sealed end faces, and raises problems in that the seals do not exhibit good sealing capabilities over a long period of time.